The long-range goal of the proposed research is to understand the process and reversible nature of cellular differentiation in mammalian systems. This problem will be approached by looking at the ability of experimentally applied biological and chemical external stimuli to mediate loss of expression of differentiated or adult functions. The aim of the initial proposal is to establish whether hormone treatment and/or virus infection (as experimental tools) can cause biochemically active, well differentiated, non-proliferating adult mammalian isolated hepatocytes in vitro, to lose liver specific expression, to proliferate, and to express fetal gene products. Since the biochemistry of liver is well known from in vivo studies and isolated hepatocytes biochemically resemble in vivo liver, isolated hepatocytes provide excellent markers for studying alterations in expression of differentiated functions. Since adult liver has the capacity to regenerate, it is feasible to assume that appropriate stimuli can trigger nonproliferating adult isolated hepatocytes to synthesize DNA, undergo mitosis, lose the expression of differentiated products, and resume some fetal functions. Once the ability to manipulate loss of differentiation in an in vitro system has been established, experiments will be carried out to determine the mechanism of this loss at a molecular level. These studies will be significant in understanding the process of differentiation in mammalian liver and, as such, will broaden our understanding of development at the cellular level.